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United States Patent |
5,620,937
|
Mulaskey
,   et al.
|
April 15, 1997
|
Pretreatment method for increasing conversion of reforming catalyst
Abstract
A pretreatment process is disclosed for increasing conversion and reducing
the fouling rate of reforming catalysts wherein the catalyst is pretreated
at a temperature from 1025.degree. F. to 1275.degree. F. in a reducing
atmosphere prior to contacting the catalyst with a hydrocarbon feed in the
presence of hydrogen.
Inventors:
|
Mulaskey; Bernard F. (Fairfax, CA);
Hise; Robert L. (Fairfield, CA);
Trumbull; Steven E. (San Leandro, CA);
Cannella; William J. (Hercules, CA);
Innes; Robert A. (San Rafael, CA)
|
Assignee:
|
Chevron Chemical Company (San Ramon, CA)
|
Appl. No.:
|
357308 |
Filed:
|
December 14, 1994 |
Current U.S. Class: |
502/66; 502/74; 502/85 |
Intern'l Class: |
B01J 029/064 |
Field of Search: |
502/66,85,74
208/140
|
References Cited
U.S. Patent Documents
4517306 | May., 1985 | Buss | 502/74.
|
4717700 | Jan., 1988 | Venkatram et al. | 502/85.
|
4822762 | Apr., 1989 | Ellig et al. | 502/66.
|
5314854 | May., 1994 | Galperin | 502/66.
|
Primary Examiner: Caldarola; Glenn A.
Assistant Examiner: Yildirim; Bekir L.
Attorney, Agent or Firm: Burns, Doane, Swecker & Mathis
Parent Case Text
This application is a continuation of application Ser. No. 08/151,814,
filed Nov. 15, 1993, now U.S. Pat. No. 5,382,353, which is a
continuation-in-part of application Ser. No. 976,786 filed Nov. 16, 1992
now abandoned.
Claims
What is claimed is:
1. A catalyst which comprises an L zeolite containing at least one Group
VIII metal, which catalyst has been treated in a gaseous environment in a
temperature range of from 1025.degree. to 1275.degree. F. while
maintaining the water level of the effluent gas below 200 ppm.
2. The catalyst of claim 1, wherein the catalyst is reduced with a reducing
gas prior to reaching the temperature of 1025.degree. F.
3. The catalyst of claim 1, wherein the catalyst is treated in the
temperature range of from 1025.degree. to 1275.degree. F. in the presence
of a reducing gas.
4. The catalyst of claim 1, wherein the catalyst has been ion exchanged
with an alkali or alkaline earth metal.
5. The catalyst of claim 4, wherein the catalyst has been ion exchanged
with sodium, cesium, rubidium, barium, strontium or calcium.
6. The catalyst of claim 1, which is in the potassium form or has been
exchanged with barium to a degree.
7. The catalyst of claim 1, wherein the catalyst comprises a Group VIII
metal.
8. The catalyst of claim 7, wherein the catalyst comprises platinum.
9. The catalyst of claim 8, wherein the amount of platinum is in the range
of from 0.1% to 5% platinum based upon the weight of the catalyst.
10. The catalyst of claim 9, wherein the amount of platinum is in the range
of from 0.1% to 1.5% by weight based upon the weight of the catalyst.
Description
BACKGROUND OF THE INVENTION
The present invention concerns a pretreatment method useful for increasing
the conversion and lowering the fouling rate of a reforming catalyst.
Catalytic reforming is a well-known process that is used for raising the
octane rating of a naphtha for gasoline. The reactions that occur during
reforming include: dehydrogenation of cyclohexanes, dehydroisomerization
of alkylcyclopentanes, dehydrocyclization of acyclic hydrocarbons,
dealkylation of alkylbenzenes, isomerization of paraffins, and
hydrocracking of paraffins. The hydrocracking reaction should be
suppressed because that reaction lowers the yield of hydrogen and lowers
the yield of liquid products.
Reforming catalysts must be selective for dehydrocyclization, in order to
produce high yields of liquid product and low yields of light gases. These
catalysts should possess good activity, so that low temperatures can be
used in the reformer. Also, they should possess good stability, so that
they can maintain a high activity and a high selectivity for
dehydrocyclization over a long period of time.
While most reforming catalysts contain platinum on an alumina support,
large-pore zeolites have been proposed as supports. These large-pore
zeolites have pores large enough for hydrocarbons in the gasoline boiling
range to pass through. Commercial application of zeolitic reforming
catalysts have thus far been very limited, although certain catalysts
comprising a large-pore zeolite containing at least one Group VIII metal
have a very high selectivity for dehydrocyclization.
It is known that reforming catalysts require pretreatment prior to
utilizing these catalysts for reforming naphtha feedstocks. For example,
U.S. Pat. No. 4,517,306 issued to Waldeen Buss on May 14, 1985 claims a
composition comprising: (a) a type L zeolite; (b) at least one Group VIII
metal; and (c) an alkaline earth metal selected from the group consisting
of barium, strontium and calcium, wherein said composition is reduced in a
hydrogen atmosphere at a temperature of from 480.degree. C. to 620.degree.
C. (896.degree. to 1148.degree. F). It is preferred that the composition
be reduced at a temperature from 550.degree. to 620.degree. C.
(1022.degree. to 1148.degree. F.).
U.S. Pat. No. 4,539,304 issued on Sep. 3, 1985 to Field discloses a
two-step pretreatment process for increasing the conversion of reforming
catalysts wherein the catalyst is first treated at a temperature of from
120.degree. C. (248.degree. F.) to 260.degree. C. (500.degree. F.) in a
reducing gas. In the second step, the temperature of the catalyst is
maintained at 370.degree. C. (698.degree. F.) to 600.degree. C.
(1112.degree. F.) in a reducing atmosphere.
U.S. Pat. No. 4,539,305 issued on Sep. 3, 1985 to Wilson et al. discloses a
pretreatment process for enhancing the selectivity and increasing the
stability of a reforming catalyst comprising a large-pore zeolite
containing at least one Group VIII metal. The catalyst is reduced in a
reducing atmosphere at a temperature of from 250.degree. C. (482.degree.)
to 650.degree. (1202.degree. F.). The reduced catalyst is subsequently
exposed to an oxygen-containing gas and then treated in a reducing
atmosphere at a temperature of from 120.degree. C. (248.degree. F.) to
260.degree. C. (500.degree. F.). Finally, the catalyst is maintained at a
temperature of from 370.degree. C. (698.degree. F.) to 600.degree. C.
(1112.degree. F.) in a reducing atmosphere. Preferably, the first
reduction step is carried out in the presence of hydrogen.
U.S. Pat. No. 5,155,075 issued to Innes et al. shows an initial catalyst
reduction at 300.degree. F. to 700.degree. F., followed by a temperature
ramp up to a final hydrogen treatment temperature between 900.degree. F.
and 1000.degree. F.
U.S. Pat. No. 5,066,632 issued on Nov. 19, 1991 to Baird et al. discloses a
process for pretreating a catalyst useful for reforming a naphtha wherein
the catalyst is calcined at temperatures in excess of 500.degree. F.,
preferably at temperatures ranging from 500.degree. F. to about
750.degree. F. in air or in atmospheres containing low partial pressures
of oxygen or in a non-reactive or inert gas such as nitrogen. The catalyst
is then contacted with a dry hydrogen-containing gas at a temperature
ranging from about 600.degree. F. to about 1000.degree. F., preferably
from about 750.degree. F. to about 950.degree. F., at a hydrogen partial
pressure ranging from about 1 atmosphere to about 40 atmospheres,
preferably from 5 atmospheres to about 30 atmospheres.
European Patent Application Publication Number 4 243,129 discloses a
catalyst activation treatment with hydrogen at temperatures from
400.degree. C. (752.degree. F.) to 800.degree. C. (1472.degree. F.),
preferably from 400.degree. C. (752.degree. F.) to 700.degree. C.
(1292.degree. F.), for a catalyst used for cracking a hydrocarbon
feedstock. The treatment pressure may vary from 100 to 5,000 MPa but is
preferably from 100 to 2,000 MPa. A carrier gas which contains 1-100% v/v,
preferably from 30-100% v/v, of hydrogen is used.
U.S. Pat. No. 4,717,700 issued to Venkatram et al discloses a method for
drying a zeolite catalyst by heating while in contact with a gas. The rate
of catalyst temperature increase is controlled so as to limit the rate of
water evolution from the catalyst and the water vapor concentration in the
gas. The gas used to heat the catalyst is gradually increased in
temperature at about 28.degree. C. per hour. The moisture level of the
effluent gas is preferably between 500 and 1500 ppm during the drying
step. The catalyst drying method with a subsequent reduction with hydrogen
wherein the temperature is raised to a maximum temperature of 450.degree.
C. is exemplified in Example 1.
Austrian Patent Specification No. 268,210 relates to a metal-charged
zeolite molecular sieve, which is suitable as a catalyst for the
conversion of hydrocarbons. Methods for preparing the catalyst are
described. It is disclosed that the catalyst prepared by such methods
usually has a high water content and that it is desirable to activate the
catalyst before use since the catalyst is sensitive to water. The
recommended activation process comprises: 1) slow heating of the catalyst
in air at 300.degree. to 600.degree. C., preferably 500.degree. C.;
followed by 2) slow heating of the catalyst from room temperature to
approximately 500.degree. C. in a current of hydrogen gas under
atmospheric pressure.
A pretreatment process of Pt-Al.sub.2 O.sub.3 catalysts in hydrogen in the
temperature range of 450.degree. C. (842.degree. F.) to 600.degree. C.
(1112.degree. F.) is disclosed in Journal of Catalysis (1979); Vol. 59, p.
138 (P. G. Menon and G. F. Froment). The effect of catalyst reduction
temperature on the conversion of n-pentane and n-hexane using Pt-Al.sub.2
O.sub.3 catalysts is disclosed. For the Pt-Al.sub.2 O.sub.3 catalyst
reduced at 400.degree. C. (752.degree. F.), hydrogenolysis is the main
reaction; whereas for the Pt-Al.sub.2 O.sub.3 catalyst reduced at
600.degree. C. (1112.degree.), the hydrogenolysis and total activity are
considerably suppressed. This reference specifically discloses the effect
of a hydrogen pretreatment process on Pt-Al.sub.2 O.sub.3 catalysts and
does not disclose the effect of hydrogen pretreatment on zeolitic
catalyst.
Additionally, the effects of hydrogen pretreatment of the Pt-Al.sub.2
O.sub.3 catalyst with respect to isomerization is disclosed. The activity
for dehydrocyclization was not increased.
Prior art processes have observed both a reduced catalytic activity and
reduced hydrogen chemisorption for catalysts which have been reduced at
temperatures in excess of 500.degree. C. Furthermore, there has been no
clear understanding of the phenomena which occur during high temperature
catalyst reduction. Thus, reduction at high temperatures may result in
strongly chemisorbed hydrogen, may cause loss of spillover hydrogen
altering the local charge transfer from the support to the metal at the
particle boundary, may induce changes in morphology of the metal
crystallite, or may affect reduction of the support resulting in the
formation of an alloy with atoms from the support.
SUMMARY OF THE INVENTION
The present invention is a process for increasing the conversion and
lowering the fouling rate of large-pore zeolitic reforming catalysts using
a pretreatment process. The catalyst is treated in a reducing gas at a
temperature of from 1025.degree. F. to 1275.degree. F.
Preferably, the pretreatment process in the range of 1025.degree. F. to
1275.degree. F. occurs in the presence of hydrogen at a pressure of from 0
to 300 psig for from 1 hour to 120 hours. Generally, the higher the
treatment temperature employed, the shorter the treatment time needed to
achieve the desired effect.
More preferably, the catalyst is reduced with dry hydrogen via
temperature-programmed steps, with the treatment of the present invention
occurring at the final temperature of from 1025.degree. F. to 1275.degree.
F. The procedure of the present invention which occurs in the temperature
range of from 1025.degree. F. to 1275.degree. F. is considered and
referred to as a "treatment" of the catalyst as opposed to a "reduction",
because the catalyst has already generally been reduced at the lower
temperatures prior to reaching the treatment temperature of the present
invention.
Among other factors, we have found that large-pore zeolitic catalysts which
have been pretreated in a reducing gas in the high temperature range of
from about 1025.degree. F. to 1275.degree. F. is found to have a lower
fouling rate and improved activity, and have a longer run life. In
particular, this catalyst exhibits a longer run life with heavier
feedstocks than with similar catalysts using other pretreatment processes.
For example, if a L zeolite catalyst is pretreated by conventional
methods, run lengths with feeds containing C.sub.9 + hydrocarbons are
generally short. The pretreatment procedure of this invention, however,
makes it practical to process feedstocks containing as much as 5-15 wt %
C.sub.9 + hydrocarbons.
Thus, in spite of the disadvantages that the prior art recognizes with
respect to high temperature catalyst reduction, the present inventors have
discovered an advantageous high temperature catalyst treatment method. In
particular, the present invention has surprisingly found that a high
temperature treatment (i.e., at 1025.degree. F. to 1275.degree. F.) will
result in a catalyst with a reduced fouling rate and sufficient catalytic
activity to yield a longer run life, particularly if the temperature
increase during reduction is performed in a gradual ramping or stepwise
fashion, and if the water content of the effluent gas is kept as low as
possible during the high temperature treatment range. Even catalysts that
are on balance non-acidic still contain a few residual acidic sites. This
high temperature treatment regimen is believed to reduce the number of
acid sites on the catalyst, and thereby reduce side reactions which lead
to the formation of coke. The improved fouling rate and conversion
activity of the catalyst also allow for more beneficial use with a heavier
feedstock.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 of the Drawing is a graphical representation of hydrogen uptake onto
catalyst as a function of temperature.
FIG. 2 of the Drawing is a graphical representation of the fouling rates
observed for different temperature treatments.
DETAILED DESCRIPTION OF THE INVENTION
In its broadest aspect, the present invention is a process for increasing
the conversion and/or lowering the fouling rate of large-pore zeolitic
reforming catalysts using a pretreatment process. This catalyst is treated
in a reducing gas at a temperature of from 1025.degree. F. to 1275.degree.
F.
Preferably, the pretreatment process occurs in the presence of hydrogen at
a pressure of from 0 to 300 psig and a temperature of from 1025.degree. F.
to 1275.degree. F. for from 1 hour to 120 hours, more preferably for at
least 2 hours, and most preferably at least 4-48 hours. More preferably,
the temperature is from 1050.degree. F. to 1250.degree. F. In general, the
length of time for the pretreatment will be somewhat dependent upon the
final treatment temperature, with the higher the final temperature the
shorter the treatment time that is needed.
For a commercial size plant, it is necessary to limit the moisture content
of the environment during the high temperature treatment in order to
prevent significant catalyst deactivation. In the temperature range of
from 1025.degree. F. to 1275.degree. F., the presence of moisture is
believed to have a severely detrimental effect on the catalyst activity,
and it has therefore been found necessary to limit the moisture content of
the environment to as little water as possible during said treatment
period, to at least less than 200 ppm.
In one embodiment, in order to limit exposure of the catalyst to water
vapor at high temperatures, it is preferred that the catalyst be reduced
initially at a temperature between 300.degree. F. and 700.degree. F. After
most of the water generated during catalyst reduction has evolved from the
catalyst, the temperature is raised slowly in ramping or stepwise fashion
to a maximum temperature between 1025.degree. F. and 1250.degree. F.
The temperature program and gas flow rates should be selected to limit
water vapor levels in the reactor effluent to less than 200 ppm and,
preferably, less than 100 ppm when the catalyst bed temperature exceeds
1025.degree. F. The rate of temperature increase to the final activation
temperature will typically average between 5.degree. and 50.degree. F. per
hour. Generally, the catalyst will be heated at a rate between 10.degree.
and 25.degree. F./h. It is preferred that the gas flow through the
catalyst bed (GHSV) during this process exceed 500 volumes per volume of
catalyst per hour, where the gas volume is measured at standard conditions
of one atmosphere and 60.degree. F. GHSV's in excess of 5000 h.sup.-1 will
normally exceed the compressor capacity. GHSV's between 600 and 2000
h.sup.-1 are most preferred.
The pretreatment process of the present invention occurs prior to
contacting the reforming catalyst with a hydrocarbon feed.
The large-pore zeolitic catalyst is generally treated in a reducing
atmosphere in the temperature range of from 1025.degree. F. to
1275.degree. F. Although other reducing gasses can be used, dry hydrogen
is preferred as a reducing gas. The hydrogen is generally mixed with an
inert gas such as nitrogen, with the amount of hydrogen in the mixture
generally ranging from 1%-99% by volume. More typically, however, the
amount of hydrogen in the mixture ranges from about 10%-50% by volume.
The reducing gas entering the reactor should contain less than 100 ppm
water. It is preferred that it contain less than 10 ppm water. In a
commercial operation, the reactor effluent may be passed through a drier
containing a desiccant or sorbent such as 4 .ANG. molecular sieves. The
dried gas containing less than 100 ppm water or, preferably, less than 10
ppm water may then be recycled to the reactor.
The feed to the reforming process is typically a naphtha that contains at
least some acyclic hydrocarbons or alkylcyclopentanes. This feed should be
substantially free of sulfur, nitrogen, metals and other known poisons.
These poisons can be removed by first using conventional hydrofining
techniques, then using sorbents to remove the remaining sulfur compounds
and water.
As mentioned above, the catalyst of the present invention exhibits a longer
run life with heavier feedstocks, e.g., containing at least 5 wt % C.sub.9
+ hydrocarbons, than similar catalysts having been subjected to a
different treatment. For example, if a L zeolite catalyst is reduced
and/or pretreated by conventional methods, run lengths with feeds
containing at least 5 wt % C.sub.9 + hydrocarbons, and typically from 5-15
wt % C.sub.9 + hydrocarbons, are comparatively short. The catalyst
obtained via the treatment of the present invention, however, makes it
quite practical to process such feedstocks containing the C.sub.9 +
hydrocarbons.
The feed can be contacted with the catalyst in either a fixed bed system, a
moving bed system, a fluidized system, or a batch system. Either a fixed
bed system or a moving bed system is preferred. In a fixed bed system, the
preheated feed is passed into at least one reactor that contains a fixed
bed of the catalyst. The flow of the feed can be either upward, downward,
or radial. The pressure is from about 1 atmosphere to about 500 psig, with
the preferred pressure being from abut 50 psig to about 200 psig. The
preferred temperature is from about 800.degree. F. to about 1025.degree.
F. The liquid hourly space velocity (LHSV) is from about 0.1 hr.sup.-1 to
about 10 hrs.sup.-1, with a preferred LHSV of from about 0.3 hr.sup.-1 to
about 5 hrs.sup.-1. Enough hydrogen is used to insure an H.sub.2 /HC ratio
of up to about 20:1. The preferred H.sub.2 /HC ratio is from about 1:1 to
about 6:1. Reforming produces hydrogen. Thus, additional hydrogen is not
needed except when the catalyst is reduced and when the feed is first
introduced. Once reforming is underway, part of the hydrogen that is
produced is recycled over the catalyst.
The catalyst is a large-pore zeolite charged with at least one Group VIII
metal. The preferred Group VIII metal is platinum, which is more selective
for dehydrocyclization and which is more stable under reforming reaction
conditions than other Group VIII metals. The catalyst should contain
between 0.1% and 5% platinum of the weight of the catalyst, preferably
from 0.1% to 1.5%.
The term "large-pore zeolite" is defined as a zeolite having an effective
pore diameter of from 6 to 15 Angstroms. The preferred pore diameter is
from 7 to 9 Angstroms. Type L zeolite, zeolite X, and zeolite Y, zeolite
beta and synthetic zeolites with the mazzite structure are thought to be
the best large-pore zeolites for this operation. Type L zeolite is
described in U.S. Pat. No. 3,216,789. Zeolite X is described in U.S. Pat.
No. 2,882,244. Zeolite beta is described in U.S. Pat. No. 3,308,069.
ZSM-4, described in U.S. Pat. No. 4,021,447, is an example of a zeolite
with the mazzite structure. Zeolite Y is described in U.S. Pat. No.
3,130,007. U.S. Pat. Nos. 3,216,789; 2,882,244; 3,130,007; 3,308,069; and
4,021,447 are hereby incorporated by reference to show zeolites useful in
the present invention. The preferred zeolite is type L zeolite.
Type L zeolites are synthesized largely in the potassium form. These
potassium cations are exchangeable, so that other type L zeolites can be
obtained by ion exchanging the type L zeolite in appropriate solutions. It
is difficult to exchange all of the original cations, since some of these
cations are in sites which are difficult to reach. The potassium may be
ion exchanged with an alkali or alkaline earth metal, such as sodium,
potassium, cesium, rubidium, barium, strontium, or calcium. Preferably,
the total amount of alkali or alkaline earth metal ions should be enough
to satisfy the cation exchange sites of the zeolite or be slightly in
excess.
An inorganic oxide can be used as a carrier to bind the large-pore zeolite.
This carrier can be natural, synthetically produced, or a combination of
the two. Preferred loadings of inorganic oxide are from 5% to 50% of the
weight of the catalyst. Useful carriers include silica, alumina,
aluminosilicates, and clays.
FIG. 1 is a plot of hydrogen uptake onto catalyst as a function of
pretreatment temperature. As can be seen from this Figure, as the
pretreatment temperature is increased, the fraction of hydrogen bound to
catalyst tends to decrease. If the hydrogen uptake onto catalyst is
reflective of the fraction of exposed Pt atoms, then one would typically
expect a decrease in activity with an increase in temperature. The extent
to which pretreating a large-pore zeolitic reforming catalyst in a
reducing environment at various temperatures affects the activity of the
catalyst will be demonstrated in Examples 1-8. The extent to which
pretreating a large-pore zeolitic reforming catalyst in a reducing
environment at various temperatures affects the fouling rate of the
catalyst will be demonstrated in Examples 9, 10, 11 and 12.
EXAMPLES
Example 1
A catalyst, consisting of 0.65% Pt on barium exchanged K-L zeolite, was
pretreated by heating the catalyst in hydrogen (P=50 psig, GHSV=9000) from
ambient temperature to 900.degree. F. at a ramp of 10.degree. F./hr and
held at 900.degree. F. for 24 hours. The temperature was adjusted to the
desired reaction temperature and n-hexane was introduced. The hydrogen to
hydrocarbon ratio was 5:1. After steady state was achieved, the
temperature was raised to the new desired reaction temperature. The
benzene production is summarized in the first line in Table 1. At
900.degree. F., the catalyst activity declined from 80% to 75%.
TABLE 1
______________________________________
Benzene Yield, wt. %
Reduction/
Treatment
Temp 800.degree. F.
830.degree. F.
860.degree. F.
900.degree. F.
______________________________________
900.degree. F.
22% 37% 55% 80-75%
1050.degree. F.
22% -- 59% 82%
1100.degree. F.
34% 54% 66% 87%
1150.degree. F.
28% -- 68% 87%
1200.degree. F.
30% 49% 70% 90%
1250.degree. F.
24% -- 57% 81%
1300.degree. F.
7% -- 30% 59%
1350.degree. F.
12% 27% 45% 73%
______________________________________
Example 2
In this case, the same catalyst as used in Example 1 was pretreated by
heating the catalyst in hydrogen (P=50 psig, GHSV=9000) from ambient
temperature to 1050.degree. F. at a ramp of 10.degree. F./hr and held at
1050.degree. F. for 3 hours. The temperature was then adjusted to the
desired reaction temperature and n-hexane was introduced to achieve a
hydrogen to hexane ratio of 5:1. The benzene production is summarized in
line 2 in Table 1.
At 860.degree. F. and 900.degree. F. reaction temperatures, the catalyst
treated at 1050.degree. F. was more active, producing more benzene, than
the catalyst reduced at 900.degree. F. In addition, the catalyst treated
at 1050.degree. F. did not exhibit deactivation at 900.degree. F. Thus,
pretreating at a high temperature of 1050.degree. F. increased the
activity and lowered the fouling rate of the catalyst.
Example 3
In this case, the same catalyst as used in Example 1 was pretreated by
heating the catalyst in hydrogen (P=50 psig, GHSV=9000) from ambient
temperature to 1100.degree. F. at a ramp of 10.degree. F./hr and held at
1100.degree. F. for 3 hours. The temperature was then adjusted to the
desired reaction temperature and n-hexane was introduced to achieve a
hydrogen to hexane ratio of 5:1. The benzene production is summarized in
line 3 in Table 1.
At all reaction temperatures, the catalyst treated at 1100.degree. F. was
more active, producing more benzene, than the catalyst treated at
900.degree. F. In addition, the catalyst reduced at 1100.degree. F. did
not exhibit deactivation at 900.degree. F. Thus, pretreating at a high
temperature of 1100.degree. F. increased the activity and lowered the
fouling rate of the catalyst.
Example 4
In this case, the same catalyst as used in Example 1 was pretreated by
heating the catalyst in hydrogen (P=50 psig, GHSV=9000) from ambient
temperature to 1150.degree. F. at a ramp of 10.degree. F./hr and held at
1150.degree. F. for 3 hours. The temperature was then adjusted to the
desired reaction temperature and n-hexane was introduced to achieve a
hydrogen to hexane ratio of 5:1. The benzene production is summarized in
line 4 in Table 1.
At the reaction temperatures of 800.degree. F., 860.degree. F. and
900.degree. F., the catalyst treated at 1150.degree. F. was more active,
producing more benzene, than the catalyst treated at 900.degree. F. In
addition, the catalyst treated at 1150.degree. F. did not exhibit
deactivation at 900.degree. F. Thus, pretreating at a high temperature of
1150.degree. F. increased the activity and lowered the fouling rate of the
catalyst.
Example 5
In this case, the same catalyst as used in Example 1 was pretreated by
heating the catalyst in hydrogen (P=50 psig, GHSV=9000) from ambient
temperature to 1200.degree. F. at a ramp of 10.degree. F./hr and held at
1200.degree. F. for 3 hours. The temperature was then adjusted to the
desired reaction temperature and n-hexane was introduced to achieve a
hydrogen to hexane ratio of 5:1. The benzene production is summarized in
line 5 in Table 1.
At all reaction temperatures, the catalyst treated at 1200.degree. F. was
more active, producing more benzene, than the catalyst reduced at
900.degree. F. In addition, the catalyst treated at 1200.degree. F. did
not exhibit deactivation at 900.degree. F. Thus, pretreating at a high
temperature of 1200.degree. F. increased the activity and lowered the
fouling rate of the catalyst.
Example 6
In this case, the same catalyst as used in Example 1 was pretreated by
heating the catalyst in hydrogen (P=50 psig, GHSV=9000) from ambient
temperature to 1250.degree. F. at a ramp of 10.degree. F./hr and held at
1250.degree. F. for 3 hours. The temperature was then adjusted to the
desired reaction temperature and n-hexane was introduced to achieve a
hydrogen to hexane ratio of 5:1. The benzene production is summarized in
line 6 in Table 1.
At the reaction temperatures of 800.degree. F., 860.degree. F. and
900.degree. F., the catalyst treated at 1250.degree. F. was more active,
producing more benzene, than the catalyst reduced at 900.degree. F. In
addition, the catalyst treated at 1250.degree. F. did not exhibit
deactivation at 900.degree. F. Thus, pretreating at a high temperature of
1250.degree. F. increased the activity and lowered the fouling rate of the
catalyst.
Example 7
The catalyst used in Example 1 was pretreated by heating the catalyst in
hydrogen (P=50 psig, GHSV-9000) from ambient temperature to 1300.degree.
F. at a ramp of 10.degree. F./hr and held at 1300.degree. F. for 3 hours.
The temperature was then adjusted to the desired reaction temperature and
n-hexane was introduced to achieve a hydrogen to hexane ratio of 5:1. The
benzene production is summarized in line 7 in Table 1.
At all reaction temperatures, the catalyst treated at 1300.degree. F. was
less active, producing less benzene, than the catalyst reduced at
900.degree. F.
Example 8
The catalyst used in Example 1 was pretreated by heating the catalyst in
hydrogen (P=50 psig, GHSV=9000) from ambient temperature to 1350.degree.
F. at a ramp of 10.degree. F./hr and held at 1350.degree. F. for 8 hours.
The temperature was then adjusted to the desired reaction temperature and
n-hexane was introduced to achieve a hydrogen to hexane ratio of 5:1. The
benzene production is summarized in line 8 in Table 1.
At all reaction temperatures, the catalyst treated at 1350.degree. F. was
less active, producing less benzene, than the catalyst reduced at
900.degree. F.
Example 9
A comparison of catalyst activity and fouling rate after reduction at
500.degree.-900.degree. F. and 500.degree.-1050.degree. F. was made as
follows.
In the first case, eighty cubic centimeters of catalyst consisting of 0.65
wt. % platinum on barium exchanged, L zeolite, 1/16 inch extrudates were
charged to a one-inch diameter reactor. The catalyst was dried by heating
to 500.degree. F. in dry nitrogen flowing at a rate of 12 cubic feet per
hour. Catalyst reduction was then initiated at 500.degree. F. by replacing
the nitrogen with dry hydrogen (preferably containing <ppm water) flowing
at the same rate. After an hour at 500.degree. F. the temperature was
raised in stepwise fashion to 900.degree. F. and maintained at 900.degree.
F. for 12 hours to complete the catalyst reduction and dryout. The
catalyst was then cooled to 800.degree. F. for feed introduction.
In the second case, the same procedure was used except that after the
initial reduction at 500.degree. F. for one hour, the temperature was
raised 10.degree. F./h to 1050.degree. F. The catalyst was then maintained
for two days at 1050.degree. F. in flowing hydrogen before cooling to
reaction temperature. The gas flow rate was 12 ft.sup.3 /hr throughout.
The feed for the catalyst performance test was a hydrotreated raffinate
from an aromatics extraction unit consisting of 8.5% C.sub.5, 59.5%
C.sub.6, 26.3% C.sub.7, and 5.8% C.sub.8.sup.+ compounds on a weight basis
This feed was also characterized as 85.8% paraffins, 6.8% naphthenes, 6.7%
aromatics, and 0.7% unknowns by weight. The test was carried out at a feed
rate of 1.6 liquid hourly space velocity, 100 psig, and a hydrogen to feed
molar ratio of 3.0. The catalyst bed temperature was adjusted as the run
progressed to maintain 42 wt % aromatics in the C.sub.5.sup.+ product. The
combined hydrogen and naphtha feedstream was treated to reduce its sulfur
content to less than 5 ppb.
The results of the test runs are shown in FIG. 2. The catalyst fouling
rates were calculated by a least squares fit of the data obtained after
200 hours on-stream. The catalyst reduced/treated at
500.degree.-1050.degree. F. had about one-fourth the fouling rate of the
catalyst reduced at 500.degree.-900.degree. F. (0.005 versus 0.020.degree.
F./h). The-start-of-run temperatures obtained by extrapolating the least
squares line back to start-of-run were 852.degree. F. and 847.degree. F.,
respectively. The yield of C.sub.5.sup.+ product was 85 LV % of feed in
both cases. Assuming the fouling rate is constant and the end-of-run
average catalyst temperature is 935.degree. F., the projected run length
is about two years for the catalyst treated at 1050.degree. F. compared to
about six months for the catalyst treated at 900.degree. F.
Example 10
This example shows that the decreased coking tendencies of the high
temperature reduced catalyst make it possible to carry out a reforming
process under previously impractical conditions. Compared to Example 9,
the liquid hourly space velocity was increased to 1.7, the
hydrogen/hydrocarbon ratio was reduced to 2.0, the pressure was increased
to 130 psig, the aromatics content of the C .sup.+ product was increased
to 72 wt. %, and a heavier feed was employed. Each of these changes would
be expected to increase fouling.
A feed containing 2.7% C.sub.5 and lighter, 8.5% C.sub.6, 49.4% C.sub.7,
30.8% C.sub.8, and 8.7% C.sub.9.sup.+ components was reformed over the
500.degree.-1050.degree. F. reduced catalyst from Example 9. The feed was
further characterized as containing 66.6% paraffins, 22.6% naphthenes,
10.5% aromatics, and 0.25% unknowns. Over a period of about 400 hours, the
fouling rate under these conditions was 0.018.degree. F./h which
corresponds to more than six months run length.
Example 11
In order to limit catalyst deactivation during the high temperature
treatment, it is important to control water vapor concentrations. This is
especially important in a commercial unit where
gas-hourly-space-velocities are limited by compressor size. It is possible
to limit the exposure of the catalyst to water vapor at high temperatures
by using dry hydrogen, measuring the moisture levels in the reactor
effluent, setting target values for each temperature range, and limiting
the rate of heatup to stay within the target moisture level ranges. A
commercial high temperature treatment was simulated in a small pilot plant
as follows.
Eighty cubic centimeters of 1/16-inch catalyst extrudates were charged to a
one-inch diameter tubular reactor. The catalyst comprised 0.65 wt %
platinum, barium exchanged L-zeolite, and a binder. The reactor was heated
by a three-zone electric furnace. Catalyst bed temperatures were measured
by six thermocouples located in an axial thermowell. The reaction system
comprised: the reactor, a chilled liquid-gas separator, a moisture
analyzer probe, a compressor, a recycle-gas drier, and a recycle gas
flowmeter. The moisture analyzer measured the moisture content in the
recycle gas before or after the drier. The drier was charged with 4 .ANG.
molecular sieves.
The unit was pressurized to 70 psig with dry nitrogen containing less than
10 ppm water. The compressor was started. Nitrogen addition was continued
in order to produce an off-gas stream and purge the system of oxygen.
After two hours, the nitrogen addition rate was reduced until there was
only a small off-gas stream. The gas circulation rate was adjusted to
maintain a gas flow over the catalyst bed corresponding to a GHSV of about
1000 h.sup.-1. The catalyst was further dried by heating the reactor to
500.degree. F. Water in the reactor effluent was removed by a drier, so
that the recycle gas contained less than 10 ppm water. The temperature was
held at 500.degree. F. until the moisture content of the reactor effluent
gas dropped below 100 ppm.
The make-up gas was then switched from nitrogen to dry hydrogen and the
unit was pressurized to 100 psig. After reaching 100 psig, the hydrogen
addition rate was adjusted to maintain a small gas bleed. The gas
circulation rate was adjusted to obtain a GHSV of about 1000 h.sup.-1.
Following hydrogen addition, there was an increase in the water content of
the reactor effluent due to catalyst reduction. This water was removed
from the recycle hydrogen stream by the recycle-gas driers. The
reactor-inlet gas contained less than 10 ppm water. The reactor
temperature was held at 500.degree. F. until the water in the reactor
effluent again dropped below 100 ppm. The reactor temperature was then
raised 10.degree. F./h to 900.degree. F. Temperature was held at
900.degree. F. until the moisture level in the reactor effluent dropped to
20 ppm. The reactor was then heated to 1100.degree. F. at a rate of
10.degree. F./h. After a 3-hour hold at 1100.degree. F., the temperature
was dropped to 800.degree. F. and the naphtha feed was introduced.
The high temperature treated catalyst was tested with several feeds at
several different conditions. When tested at the conditions used in
Example 9, but with a heavier feed, the fouling rate was 0.007.degree.
F./h compared to 0.025.degree. F./h for the same catalyst reduced in the
temperature range of from 500.degree. to 900.degree. F.
Example 12
A potassium L-zeolite catalyst also surprisingly benefits from a high
temperature hydrogen treatment. Platinum was loaded onto a bound, 20-40
mesh, K-L zeolite support using the incipient wetness impregnation method
and an aqueous Pt(NH.sub.3)Cl.sub.2 -H.sub.2 O solution. The impregnated
material was oven-dried at 120.degree. F. overnight and calcined at
500.degree. F. for four hours.
In three separate experiments, one-gram of the calcined material was loaded
into a 3/16" I.D. tubular microreactor. In each case, the catalyst was
dried by heating to 500.degree. F. in nitrogen flowing at a rate of 550
cc/min. In the first experiment, the catalyst was reduced in 550 cc/min of
hydrogen while the reactor temperature was heated from 500.degree. to
900.degree. F. at a rate of 10.degree. F./h. In the second and third
experiments, the activation procedure was the same except that the final
temperatures were 1100.degree. and 1150.degree. F., respectively. The
catalyst samples were held at their peak temperature for three hours, then
cooled to 875.degree. F. for testing.
A C.sub.5 -C.sub.8 raffinate stream from an aromatics extraction unit was
reacted in the presence of hydrogen over each catalyst sample. Reactor
effluent analyses were obtained by gas chromatography. Conversion and
selectivity were calculated from the feed and product analyses. Table 2
shows that the stability of the Pt-K-L zeolite catalyst was significantly
improved by high temperature reduction. Conversion after about six days
on-stream was significantly higher for the catalysts treated at
1100.degree. or 1150.degree. F. than when the reduction temperature was
limited to 900.degree. F. "Conversion" refers to the conversion of
C.sub.6.sup.+ feed components and "selectivity" is the selectivity for
aromatics and hydrogen production. Both are calculated on a weight basis.
TABLE 2
______________________________________
Catalyst Reduction
Hours on Conversion
Selectivity
Temperature Stream Wt % Wt %
______________________________________
500-900.degree. F.
3 62.3 87.3
145 36.1 89.0
500-1100.degree. F.
6 61.8 88.5
146 50.5 90.9
500-1150.degree. F.
5 53.7 89.0
147 44.8 90.6
______________________________________
Run conditions:
WHSV = 4.4, H.sub.2 /HC = 5.0, Temp. = 875.degree. F., Pres. = 50 psig
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